A brushed direct current motor (hereinafter called the “DC motor”) has been widely used in a vehicle. For example, the DC motor is used in an air-mixing damper and a mode damper of an air conditioner of a vehicle. The air-mixing damper allows temperature adjusting, and the mode damper allows air outlet switching. There is a need to accurately detect a rotation state (e.g., rotation angle, rotation direction, rotation speed) of the DC motor in order to accurately control such dampers.
In a conventional method, a rotation state of a DC motor is detected by using a large-scale sensor such as a rotary encoder or a potentiometer. Such a conventional method using the large-scale sensor can be employed to detect a rotation state of a vehicular DC motor.
However, the use of the large-scale sensor in a vehicle increases the size, weight, and cost of the vehicle. Therefore, there is an increased demand for a sensorless method to detect a rotation state of a DC motor without using the large-scale sensor.
Some sensorless methods have been proposed. In one sensorless method, a rotation state of a DC motor is detected by detecting and counting a surge pulse that is generated when the contact between a commutator and a brush changes. However, this method based on the surge pulse has the following disadvantage. The electromotive force of the motor is small, when the motor rotates at low speed, for example, when the motor starts and stops. Accordingly, the surge pulse is small, when the motor rotates at low speed. Therefore, it becomes more difficult to detect the surge pulse, as the rotation speed of the motor becomes lower. As a result, a detection error may occur.
JP 2003-111465 A discloses another sensorless method. In the method disclosed in JP 2003-111465 A, a resistor is connected between two of commutator segments of a commutator so that the resistor can be connected in parallel to a phase coil connected between the two commutator segments, and a rotation pulse is detected based on, an electric current flowing between the commutator segments.
In the method disclosed in JP 2003-111465 A, when a direct current is supplied through brushes to a motor circuit (i.e., armature coil side circuit), the current flowing between the brushes changes periodically according to a rotation angle of a motor. Since the rotation pulse is detected based on a change in the current, the method disclosed in JP 2003-111465 A can improve a detection accuracy compared to the method based on the surge pulse.
However, the method disclosed in JP 2003-111465 A has the following disadvantage. The change in the current flowing to the motor circuit causes a variation in torque of the motor. Due to the torque variation, the motor produces noise or an apparatus driven by the motor produces noise.
Further, even in the method disclosed in JP 2003-111465 A, as the rotation speed of the motor becomes lower, the change in the current becomes smaller. Therefore, a detection error may occur.